feat: add native clipboard support for screenshot paste

- Add Tauri clipboard-manager plugin for native clipboard access
- Fall back to native clipboard when WebView clipboard API returns empty
- Convert RGBA image data to PNG via canvas for saving
- Allow sending messages with attachments only (no text required)
- Log attached files to output with 📎 emoji
- Update send button to enable when attachments exist

Co-Authored-By: Naomi Carrigan <commits@nhcarrigan.com>

src-tauri/capabilities/default.json

@@ -13,6 +13,8 @@
     "notification:default",
     "notification:allow-is-permission-granted",
     "notification:allow-request-permission",
-    "notification:allow-notify"
+    "notification:allow-notify",
+    "clipboard-manager:default",
+    "clipboard-manager:allow-read-image"
   ]
 }